Pontiac G6 2010 Owner's Manual page 280

To remove the fuel cap, turn it slowly counterclockwise.
The fuel cap has a spring in it; if the cap is released
too soon, it will spring back to the right.
While refueling, hang the tethered fuel cap from the
hook on the fuel door.
WARNING:
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Fuel can spray out on you if you open the fuel cap
too quickly. If you spill fuel and then something
ignites it, you could be badly burned. This spray
can happen if the tank is nearly full, and is more
likely in hot weather. Open the fuel cap slowly and
wait for any hiss noise to stop. Then unscrew the
cap all the way.
6-12
Be careful not to spill fuel. Do not top off or overfill the
tank, and wait a few seconds after you have finished
pumping before removing the nozzle. Clean fuel from
painted surfaces as soon as possible. See Washing Your
Vehicle on page 6-110.
When replacing the fuel cap, turn it clockwise until it
clicks. Make sure the cap is fully installed. The diagnostic
system can determine if the fuel cap has been left off or
improperly installed. This would let fuel evaporate into
the atmosphere. See Malfunction Indicator Lamp on
page 4-34.
The CHECK GAS CAP message displays on the Driver
Information Center (DIC) if the fuel cap is not properly
installed. See DIC Warnings and Messages on
page 4-42 for more information.